Functional Test Sequences as a Source for Partially Functional Launch-on-Shift Tests

Scan-based tests were shown to create nonfunctional operation conditions that can cause a fault-free circuit to exhibit the appearance of a delay fault. Scan-based tests that are extracted from functional test sequences avoid nonfunctional operation conditions during their functional capture cycles and are referred to as functional scan-based tests. This brief observes that the stuck-at fault coverage of functional single-cycle tests is significantly higher than the transition fault coverage of functional launch-on-capture (LOC) tests that are extracted from the same sequences. To bridge the fault coverage gap, this brief describes a procedure that extracts both functional LOC and close-to-functional launch-on-shift (LOS) tests together from functional test sequences. The LOS tests have the unique property that their functional capture cycles consist of functional single-cycle tests. Experimental results for benchmark circuits show significant reductions in the fault coverage gap between stuck-at and transition faults.